This invention relates generally to non-destructive testing and, more particularly, to ultrasound inspection of composite parts.
Ultrasonic inspection techniques are used in many applications where non-destructive evaluation of a workpiece is required. One application of such ultrasonic inspection is in the inspection of composite fiber reinforced aircraft propeller blades. Such blades are typically formed from a plurality of layers of composite fibers (graphite, boron or S-glass, for example) laid over each other and adhesively bonded. Any separation of the fiber layers due to an incomplete bond or void in the blade may detrimentally affect blade strength. Ultrasonic inspection techniques can be used to identify and locate such flaws in a composite fiber reinforced blade. Additionally, ultrasound inspection techniques can be used to inspect composite aircraft engine parts, such as, but not limited to, composite ducts.
One technique of ultrasound inspection is sometimes referred to as bottom echo detection. By measuring the part thickness and knowing the attenuation characteristics of the part being inspected, one can compute an expected arrival time for the echo. If the echo is received earlier than expected then the ultrasound sound wave encountered an interface causing a reflection of the sound wave. This early reflection is typically caused by a delamination of the lamination layers indicating a flaw in the object. An echo received at the expected time is indicative of an absence of a delamination. This technique, however, relays on the absence or presence of the echoed sound wave and is therefore commonly called a “go/no go” type of inspection. At least one disadvantage of using a “go/no go” type of inspection is that small micro-cracks may be undetected.